The present invention relates to protective relay circuits for use in AC power distribution systems and more particularly to ground distance relays for transmission line protection.
Ground distance relays are utilized in protective systems for AC power transmission lines to detect faults between a phase and ground within a protected zone or portion of the transmission line. If such a fault is detected within the zone or reach of the relay, the ground distance relay may generate a signal to operate a circuit breaker in such a manner as to trip only the faulted phase of the transmission line.
Assuming a polyphase transmission line comprising phases A, B, and C, each particular ground distance relay should only detect faults related to a particular phase. For example, a ground distance relay for phase A should only detect phase A-to-ground faults. Furthermore, the ground distance relay for phase A should not be affected by a phase B-to-ground fault, a phase C-to-ground fault, or any phase-to-phase fault. The single phase discriminating detection operations are desired so that the fault related to the single phase will not cause a three pole trip of the circuit breaker. This is to be contrasted with a phase relay which detects faults involving more than one electrical phase, again within a designated zone or reach; and, upon detection of such a fault, generates a signal which will cause a three pole trip of the circuit breaker.
As indicated above, each particular ground distance relay should only detect faults within its protected zone or reach. The parameters of a ground distance relay are commonly selected to correspond to the parameters of the transmission line. For example, the parameters may be selected to provide a forward reach that may correspond to the forward distance of the zone of the transmission line under the protection of the particular relay. It is desired that the ground distance relay operate within its selected forward reach, thereby confining the protection of a particular ground distance relay to a selected zone within a protected system. The forward reach commonly defines the protected zone within the transmission line desired to be protected by the ground distance relay.
The desired operation of a ground distance relay may be hindered by the inability of the relay to distinguish between internal and external faults. Internal faults are those faults which occur within the protected zone or reach of the relay, whereas external faults are those that occur outside of the zone or relay reach. Undesired operation in response to external faults beyond the reach of the relay is sometimes referred to as overreaching.
The desired operation of a ground distance relay in single pole tripping schemes may also be hindered by the inability to distinguish between phase-to-ground faults and phase-to-phase faults. For example, heavy load flow in long lines can cause the impedances seen by a ground distance relay for a phase-to-phase fault to appear within its operating zone for one of the two operating phases. One phase tends to overreach while the other phase tends to underreach.
An additional problem which can adversely affect the operation of ground distance relays is operation on reverse type faults; that is, a fault occurring behind the ground distance relay. Such faults could be caused by, for example, a fault occurring behind the relay where a series capacitor is situated between the fault and the relay location such that the operate signal tends to reverse compared to a "normal" external fault. Further, in applications with heavy load and long relay reaches, the unfaulted phase relays have a tendency to operate for some unbalanced faults behind the relay.
Still another problem which can adversely affect the operation of a ground distance relay scheme is the generation of a three pole tripping signal upon occurrence of an inter-circuit fault (i.e., a double line to ground fault involving different phases, each phase associated with a different circuit). For example, for an A to ground fault on one circuit and a B to ground fault on a second circuit, both faults at the remote end of a double circuit line, it is desirable to block operation of the ground distance relays so that a single pole permissive trip signal without phase identification can be transmitted from the faulted end without causing an incorrect three pole trip. In the overall protective scheme, receipt of a single pole permissive trip signal without phase identification will not permit the phase relays to initiate a three pole trip but will allow the ground distance relays to initiate tripping of their respective poles of the circuit breaker when any one of them operates. Thus, when the faulted phase(s) is cleared at the remote end, the fault appears as a single line to ground fault and the ground distance relay responds to trip the faulted phase.
The desired operation of a protective relay system may also be degraded by non-operation of the ground distance relay upon the occurrence of a severe, close-in, double line to ground fault. Such a fault may severely impact the stability of the power system if not cleared very rapidly. Thus, the ground distance relays are desirable to operate for such a fault to increase the speed of clearing.
Accordingly, it is an object of the present invention to provide improved discrimination between internal and external faults thereby increasing the reliability of ground distance relay operation and the security of the protection system.
Another object of the present invention is to minimize erroneous operation of the ground distance relay due to operation on reverse type faults.
It is still another object of the present invention to prevent overreaching by the ground distance relay on single line-to-ground and double line-to-ground faults, even in the difficult case of faults having a substantial fault resistance.
It is yet another object of the present invention to prevent operation of the ground distance relay on a particular phase when the fault occurs on another phase.
It is still a further object of the present invention to inhibit operation of the ground relay on double line-to-ground faults in order to insure tripping of only the faulted phase during remote end inter-circuit type faults.
It is yet another object of the present invention to cause operation of the ground distance relay on severe close-in, double line-to-ground faults in order to improve the speed of operation of the relay system.
It is a further object of the present invention to prevent operation of the ground distance relay associated with unfaulted phases on severe close-in single line-to-ground faults, even in the difficult case of faults with substantial fault resistance.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.